The present invention relates to a spray dampening system. More particularly, the present invention relates to a control technique for the spray bar of a pulsed spray dampener.
Various systems have been proposed in the past for applying a fluid to the rollers of printing presses. These fluids may be used, for example, for dampening or cleaning the rollers, or for preventing printing offset. One type system dampens the rollers by spraying a fluid mist from nozzles positioned adjacent the rollers. Typically, a plurality of nozzles are aligned in a spray bar. It is often desirable to vary the spray output of a spray dampener in response to certain operating parameters such as press speed and printing density.
U.S. Pat. No. 4,469,024 to Schwartz et al relates to a pulsed spray dampener is dispensed is controlled by a measured press speed. In the disclosed embodiment, a speed sensor generates a sinusoidal sensor signal having a frequency related to the press speed. A pulse width modulator receives the sinusoidal sensor signal and generates a square wave control signal wherein the pulse duration is maintained constant. The time between pulses in the square wave control signal is varied as a function of press speed. The control signal is converted to pneumatic pulses used operate air-actuated valves which supply fluid to the spray nozzles.
Another spray dampener system is disclosed in U.S. Pat. No 4,649,818 to Switall et al. A speed sensor provides a sensor signal to a master controller which, on the basis of the sensor signal value, selects one of a plurality of oscillating electrical signals having discrete frequencies. The selected frequency signal is supplied to a monostable which produces a fixed-length pulse in response to the leading edge of each cycle of the frequency signal. The monostable pulses are then used to operate spray nozzle solenoids. The width of the monostable pulse may be adjusted manually.
U.S. Pat. No. 3,926,115 to Alsop discloses a spray dampening apparatus wherein the fluid output may be temporarily varied by partial or complete interruption of the spray. A solid obstructor may be placed in the spray path or a deflecting air blast may be used to vary the spray output. In the spray dampener disclosed in U.S. Pat. No. 3,924,531 to Klinger, the output spray may be controlled by varying the position of various mechanical members.
All of the known spray dampeners have had several drawbacks. For example, pulsed spray dampeners often encounter difficulties which lead to poor spray patterns and the like. In a system wherein the amount of dampening fluid is varied by changing the "ON" time of the spray nozzles, control of the dampening fluid output during low speed press operation is restricted by physical limitations in the spray nozzles, valves, and the like. Additionally, in systems wherein the "OFF" time of the spray nozzles is controlled, the controller is limited by the possibility of drying when there are long periods of time between spray pulses. Systems using a physical technique to vary spray output encounter difficulties in obtaining a proper spray pattern. Accordingly, there exists a need for a spray dampening system which overcomes the difficulties confronted in earlier spray dampening systems.